Surgical system for positioning prosthetic component and/or for constraining movement of surgical tool

ABSTRACT

A surgical system includes a surgical tool configured to be coupled to a cutting element, a force system configured to provide at least some force to the surgical tool, and a controller programmed to generate control signals. The control signals cause the force system to provide a first constraint on a user&#39;s manual movement of the surgical tool when the cutting element is a first cutting element and provide a second constraint, different from the first constraint, on a user&#39;s manual movement of the surgical tool when the cutting element is a second cutting element.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/894,071, filed Sep. 29, 2010, which claims the benefit of andpriority to U.S. Provisional Patent Application Ser. No. 61/278,066,filed Oct. 1, 2009; U.S. Provisional Patent Application Ser. No.61/339,460, filed Mar. 4, 2010; U.S. Provisional Patent Application Ser.No. 61/339,756, filed Mar. 9, 2010; and U.S. Provisional PatentApplication Ser. No. 61/401,209, filed Aug. 9, 2010, each of which ishereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of Invention

The present invention relates generally to robotic systems and, moreparticularly, to surgical systems for orthopedic joint replacementsurgery.

Description of Related Art

Robotic systems are often used in applications that require a highdegree of accuracy and/or precision, such as surgical procedures orother complex tasks. Such systems may include various types of robots,such as autonomous, teleoperated, and interactive.

Interactive robotic systems are preferred for some types of surgery,such as joint replacement surgery, because they enable a surgeon tomaintain direct, hands-on control of the surgical procedure while stillachieving a high degree of accuracy and/or precision. For example, inknee replacement surgery, a surgeon can use an interactive, hapticallyguided robotic arm in a passive manner to sculpt bone to receive a jointimplant, such as a knee implant. To sculpt bone, the surgeon manuallygrasps and manipulates the robotic arm to move a cutting tool (such as aburr) that is coupled to the robotic arm to cut a pocket in the bone. Aslong as the surgeon maintains a tip of the burr within a predefinedvirtual cutting boundary defined, for example, by a haptic object, therobotic arm moves freely with low friction and low inertia such that thesurgeon perceives the robotic arm as essentially weightless and can movethe robotic arm as desired. If the surgeon attempts to move the tip ofthe burr to cut outside the virtual cutting boundary, however, therobotic arm provides haptic (or force) feedback that prevents orinhibits the surgeon from moving the tip of the burr beyond the virtualcutting boundary. In this manner, the robotic arm enables highlyaccurate, repeatable bone cuts. When the surgeon manually implants aknee implant (such as a patellofemoral component) on a correspondingbone cut the implant will generally be accurately aligned due to theconfiguration of and interface between the cut bone and the kneeimplant.

The above-described interactive robotic system, though useful for kneereplacement surgery, it is not optimally suited for types of surgery,such as hip replacement surgery, that require the use of multiplesurgical tools having different functions (e.g., reaming, impacting),different configurations (e.g., straight, offset), and differentweights. A system designed to accommodate a variety of tools may beprohibitively complex and require multiple end effectors, and removingand attaching different types of tools to the robotic arm during asurgical procedure could increase the time to perform the procedure.Additionally, in hip replacement surgery, in addition to maintaining anappropriate cutting boundary, angular orientation of surgical tools andimplants is important. For example, in conventional hip replacementsurgery, the surgeon uses a hemispherical reamer to resurface apatient's acetabulum, which is a cup-shaped socket in the pelvis. Then,a corresponding cup-shaped implant (an acetabular cup), is attached to adistal end of an impactor tool. The surgeon implants the acetabular cupinto the reamed socket by repeatedly striking a proximal end of theimpactor tool with a mallet. Angular orientation of both the reamedsocket and the implanted acetabular cup is important because incorrectindividual and/or relative orientation can result in misalignment of theacetabular cup to the appropriate version and inclination angles of thepatient's acetabular anatomy. Misalignment can lead to post-operativeproblems, including joint dislocation, impingement of the femur on theacetabular cup at the extreme ranges of motion of the femur, andaccelerated wear of the acetabular cup due to improper loading of thefemoral head-to-acetabular cup interface. Alignment is also important tomaintain correct leg length and medial/lateral offset. Finally,impacting the acetabular cup into the reamed socket generates highimpact forces that could potentially damage a robotic arm designed forhighly accurate and/or precise operation.

In view of the foregoing, a need exists for an improved robotic surgicalsystem and components thereof.

SUMMARY

According to an aspect of the present invention, a surgical system forpositioning a prosthetic component on an anatomy of a patient includes asurgical tool configured to engage the prosthetic component, a forcesystem configured to provide at least some force to the surgical tool,and a controller programmed to compare a target pose of the prostheticcomponent and an actual pose of the prosthetic component engaged by thesurgical tool and generate control signals that cause the force systemto allow movement of the surgical tool within a range of movement andprovide haptic feedback to constrain a user's ability to manually movethe surgical tool beyond the range of movement. The haptic feedbackresists movement of the surgical tool by the user that would causesubstantial deviation between at least one aspect of the actual pose ofthe prosthetic component and a corresponding aspect of the target poseof the prosthetic component. The controller is programmed to generatecontrol signals that cause the force system to maintain the hapticfeedback as the user implants the prosthetic component on the anatomy.

According to another aspect, a surgical system includes a surgical toolconfigured to be coupled to a cutting element, a force system configuredto provide at least some force to the surgical tool, and a controllerprogrammed to generate control signals that cause the force system toprovide a first constraint on a user's manual movement of the surgicaltool when the cutting element is a first cutting element and provide asecond constraint, different from the first constraint, on a user'smanual movement of the surgical tool when the cutting element is asecond cutting element.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated and constitute a partof this specification, illustrate embodiments of the invention andtogether with the description serve to explain aspects of the invention.

FIG. 1A is a perspective view of a femur and a pelvis.

FIG. 1B is a perspective view of a hip joint formed by the femur andpelvis of FIG. 1A.

FIG. 2A is an exploded perspective view of a femoral component and anacetabular component for a total hip replacement procedure.

FIG. 2B is a perspective view illustrating placement of the femoralcomponent and acetabular component of FIG. 2A in relation to the femurand pelvis of FIG. 1A, respectively.

FIG. 3A is a perspective view of an embodiment of a surgical system.

FIG. 3B is a perspective view of an embodiment of a robotic arm of thesurgical system of FIG. 3A.

FIG. 4A is a perspective view of an embodiment of an end effectorcoupled to an embodiment of an operating member.

FIG. 4B is a cross-sectional view of the end effector and operatingmember of FIG. 4A.

FIG. 4C is a perspective view of a shaft of the operating member of FIG.4A.

FIG. 4D is a cross sectional view of the shaft of FIG. 4C taken alongline N-N.

FIG. 5A is a cross-sectional view of an embodiment of a coupling deviceof the end effector of FIG. 4A in a release position.

FIG. 5B is a cross-sectional view of the coupling device of FIG. 5A in aconnect position.

FIG. 5C is a perspective view of an embodiment of a receiving portion ofthe end effector of FIG. 4A.

FIG. 5D is a perspective view of an embodiment of a retaining member ofthe coupling device of FIG. 5A.

FIG. 5E is a perspective view of a slide member of the coupling deviceof FIG. 5A.

FIG. 6A is a perspective view of the end effector of FIG. 4A coupled toanother embodiment of an operating member.

FIG. 6B is a cross-sectional view of the end effector and operatingmember of FIG. 6A.

FIG. 6C is a perspective view of a shaft of the operating member of FIG.6A.

FIG. 7A is a cross-sectional view of the end effector of FIG. 4A coupledto another embodiment of an operating member in a seated position.

FIG. 7B is a cross-sectional view of the end effector and operatingmember of FIG. 7A in an extended position.

FIG. 8A is a cross-sectional view of the coupling device of the endeffector of FIG. 7A in a release position.

FIG. 8B is a cross-sectional view of the coupling device of FIG. 8A in aconnect position.

FIG. 9A is a perspective view of the end effector of FIG. 4A coupled toanother embodiment of an operating member.

FIG. 9B is a cross-sectional view of the end effector and operatingmember of FIG. 9A.

FIG. 10A illustrates how a surgeon holds the end effector and operatingmember of FIG. 4A for a reaming an acetabulum of a patient.

FIG. 10B illustrates how a surgeon uses the end effector and operatingmember of FIG. 7A to impact an acetabular cup into a reamed acetabulumof a patient.

FIG. 11 illustrates an embodiment of steps of a hip replacementprocedure.

FIG. 12 is a cross-sectional view of an embodiment of a first cuttingelement and a second cutting element.

FIG. 13A illustrates an embodiment of a first constraint.

FIG. 13B illustrates an embodiment of a second constraint.

FIGS. 14A-14G illustrate embodiments of a computer display for useduring a surgical procedure.

FIG. 15 illustrates an embodiment of a virtual brake.

FIG. 16A illustrates an embodiment of a virtual brake that isdisengaged.

FIG. 16B illustrates an embodiment of a virtual brake that is engaged.

FIG. 17A illustrates an embodiment of an instrumented linkage.

FIG. 17B illustrates an embodiment of the instrumented linkage of FIG.17A in a parking configuration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Presently preferred embodiments of the invention are illustrated in thedrawings. An effort has been made to use the same or like referencenumbers throughout the drawings to refer to the same or like parts.Although this specification refers primarily to a robotic arm fororthopedic hip replacement, it should be understood that the subjectmatter described herein is applicable to other types of robotic systems,including those used for surgical and non-surgical applications, as wellas to other joints of the body, such as, for example, a shoulder joint.

Overview

The hip joint is the joint between the femur and the pelvis andprimarily functions to support the weight of the body in static (forexample, standing) and dynamic (for example, walking) postures. FIG. 1Aillustrates the bones of a hip joint 10, which include a pelvis 12(shown in part) and a proximal end of a femur 14. The proximal end ofthe femur 14 includes a femoral head 16 disposed on a femoral neck 18.The femoral neck 18 connects the femoral head 16 to a femoral shaft 20.As shown in FIG. 1B, the femoral head 16 fits into a concave socket inthe pelvis 12 called the acetabulum 22, thereby forming the hip joint10. The acetabulum 22 and femoral head 16 are both covered by articularcartilage that absorbs shock and promotes articulation of the joint 10.

Over time, the hip joint 10 may degenerate (for example, due toosteoarthritis) resulting in pain and diminished functionality. As aresult, a hip replacement procedure, such as total hip arthroplasty orhip resurfacing, may be necessary. During hip replacement, a surgeonreplaces portions of a patient's hip joint 10 with artificialcomponents. In total hip arthroplasty, the surgeon removes the femoralhead 16 and neck 18 and replaces the natural bone with a prostheticfemoral component 26 comprising a head 26 a, a neck 26 b, and a stem 26c (shown in FIG. 2A). As shown in FIG. 2B, the stem 26 c of the femoralcomponent 26 is anchored in a cavity the surgeon creates in theintramedullary canal of the femur 14. Alternatively, if disease isconfined to the surface of the femoral head 16, the surgeon may opt fora less invasive approach in which the femoral head is resurfaced (e.g.,using a cylindrical reamer) and then mated with a prosthetic femoralhead cup (not shown). Similarly, if the natural acetabulum 22 of thepelvis 12 is worn or diseased, the surgeon resurfaces the acetabulum 22using a reamer and replaces the natural surface with a prostheticacetabular component 28 comprising a hemispherical shaped cup 28 a(shown in FIG. 2A) that may include a liner 28 b. To install theacetabular component 28, the surgeon connects the cup 28 a to a distalend of an impactor tool and implants the cup 28 a into the reamedacetabulum 22 by repeatedly striking a proximal end of the impactor toolwith a mallet. If the acetabular component 28 includes a liner 28 b, thesurgeon snaps the liner 28 b into the cup 28 a after implanting the cup28 a. Depending on the position in which the surgeon places the patientfor surgery, the surgeon may use a straight or offset reamer to ream theacetabulum 22 and a straight or offset impactor to implant theacetabular cup 28 a. For example, a surgeon that uses a postero-lateralapproach may prefer straight reaming and impaction whereas a surgeonthat uses an antero-lateral approach may prefer offset reaming andimpaction.

Exemplary Robotic System

A surgical system can be configured according to the present inventionto perform hip replacement, as well as other surgical procedures. Asshown in FIG. 3A, an embodiment of a surgical system 5 for surgicalapplications according to the present invention includes a computerassisted navigation system 7, a tracking device 8, a display device 9(or multiple display devices 9), and a robotic arm 30.

The robotic arm 30 can be used in an interactive manner by a surgeon toperform a surgical procedure on a patient, such as a hip replacementprocedure. As shown in FIG. 3B, the robotic arm 30 includes a base 32,an articulated arm 34, a force system (not shown), and a controller (notshown). A surgical tool (e.g., an end effector 40 having an operatingmember) is coupled to the articulated arm 34, and the surgeonmanipulates the surgical tool by grasping and manually moving thearticulated arm 34 and/or the surgical tool.

The force system and controller are configured to provide control orguidance to the surgeon during manipulation of the surgical tool. Theforce system is configured to provide at least some force to thesurgical tool via the articulated arm 34, and the controller isprogrammed to generate control signals for controlling the force system.In one embodiment, the force system includes actuators and abackdriveable transmission that provide haptic (or force) feedback toconstrain or inhibit the surgeon from manually moving the surgical toolbeyond predefined virtual boundaries defined by haptic objects asdescribed, for example, in U.S. patent application Ser. No. 11/357,197(Pub. No. US 2006/0142657), filed Feb. 21, 2006, and/or U.S. patentapplication Ser. No. 12/654,591, filed Dec. 22, 2009, each of which ishereby incorporated by reference herein in its entirety. In a preferredembodiment the surgical system is the RIO® Robotic Arm InteractiveOrthopedic System manufactured by MAKO Surgical Corp. of FortLauderdale, Fla. The force system and controller are preferably housedwithin the robotic arm 30.

The tracking device 8 is configured to track the relative locations ofthe surgical tool (coupled to the robotic arm 34) and the patient'sanatomy. The surgical tool can be tracked directly by the trackingdevice 8. Alternatively, the pose of the surgical tool can be determinedby tracking the location of the base 32 of the robotic arm 30 andcalculating the pose of the surgical tool based on joint encoder datafrom joints of the robotic arm 30 and a known geometric relationshipbetween the surgical tool and the robotic arm 30. In particular, thetracking device 8 (e.g., an optical, mechanical, electromagnetic, orother known tracking system) tracks (or enables determination of) thepose (i.e., position and orientation) of the surgical tool and thepatient's anatomy so the navigation system 7 knows the relativerelationship between the tool and the anatomy.

In operation, a user (e.g., a surgeon) manually moves the robotic arm 30to manipulate the surgical tool (e.g., the end effector 40 having anoperating member) to perform a surgical task on the patient, such asbone cutting or implant installation. As the surgeon manipulates thetool, the tracking device 8 tracks the location of the surgical tool andthe robotic arm 30 provides haptic (or force) feedback to limit thesurgeon's ability to move the tool beyond a predefined virtual boundarythat is registered (or mapped) to the patient's anatomy, which resultsin highly accurate and repeatable bone cuts and/or implant placement.The robotic arm 30 operates in a passive manner and provides hapticfeedback when the surgeon attempts to move the surgical tool beyond thevirtual boundary. The haptic feedback is generated by one or moreactuators (e.g., motors) in the robotic arm 30 and transmitted to thesurgeon via a flexible transmission, such as a cable drive transmission.When the robotic arm 30 is not providing haptic feedback, the roboticarm 30 is freely moveable by the surgeon and preferably includes avirtual brake that can be activated as desired by the surgeon. Duringthe surgical procedure, the navigation system 7 displays images relatedto the surgical procedure on one or both of the display devices 9.

End Effector

A surgical tool has been developed that can be configured, for example,to work with the robotic arm 30 while allowing modification andperformance of different functions. FIGS. 4A-4C show an embodiment of asurgical tool according to the present invention. In this embodiment,the tool is an end effector 40 configured to be mounted to an end of therobotic arm 30. The end effector 40 includes a mounting portion 50, ahousing 60, a coupling device 70, and a release member 80. The endeffector 40 is configured to individually and interchangeably supportand accurately position multiple operating members relative to therobotic arm 30. In FIGS. 4A-4C, the end effector 40 is coupled to anoperating member 100.

The mounting portion (or mount) 50 preferably couples the end effector40 to the robotic arm 30. In particular, the mounting portion 50 extendsfrom the housing 60 and is configured to couple the end effector 40 to acorresponding mounting portion 35 of the robotic arm 30 using, forexample, mechanical fasteners, such that the mounting portions are fixedrelative to one another. The mounting portion 50 can be attached to thehousing 60 or formed integrally with the housing 60 and is configured toaccurately and repeatably position the end effector 40 relative to therobotic arm 30. In one embodiment, the mounting portion 50 is asemi-kinematic mount as described in U.S. patent application Ser. No.12/644,964, filed Dec. 22, 2009, and hereby incorporated by referenceherein in its entirety.

The housing 60 is configured to receive the operating member 100 and toprovide a user interface for the surgeon. For example, as shown in FIGS.10A and 10B, the surgeon grasps the housing 60 to manipulate the endeffector 40 to perform a task with the operating member 100. In thisembodiment, the housing 60 is a hollow elongated cylinder having acentral axis A-A, a proximal end 60 a, and a distal end 60 b.

Referring to FIG. 4B, to install the operating member 100 in the endeffector 40, the surgeon inserts a proximal end 100 a of a shaft 110 ofthe operating member 100 into the distal end 60 b of the housing 60,slides the shaft 110 in a direction T1, and actuates the release member80, which moves the coupling device 70 to a “release” position andenables the shaft 110 to be fully received in the housing 60. When theshaft 110 extends from the proximal end 60 a of the housing 60 by anappropriate amount, the surgeon releases the release member 80, whichmoves the coupling device 70 to a “connect” position and couples theshaft 110 to the housing 60. Once the shaft 110 is coupled to thehousing, additional equipment can be attached to the shaft 110, such asa drive motor 112, a cutting element 116, or other component of theoperating member 100.

To remove the operating member 100 from the end effector 40, the surgeonremoves the drive motor 112 and cutting element 116 and actuates therelease member 80, which moves the coupling device 70 to the releaseposition. The surgeon then slides the shaft 110 in a direction T2 untilthe operating member 100 clears the distal end 60 b of the housing 60.

The end effector 40 may include a receiving portion 62 that permits onlydesired movement of the operating member 100. The receiving portion 62is disposed within the housing 60. The receiving portion 62 isconfigured to receive at least a portion of the operating member 100 soas to permit rotation of the operating member 100 relative to thehousing 60 while constraining movement of the operating member 100 in aradial direction R of the operating member 100 (shown in FIG. 4D). Forexample, as shown in FIGS. 5A and 5C, the receiving portion 62 includesa flange 64 that is affixed to the housing 60 (e.g., using mechanicalfasteners) and a cylindrical portion 66 through which the operatingmember 100 extends. Although the receiving portion 62 is fixed relativeto the housing 60 via the flange 64, the operating member 100 is notconnected to the receiving portion 62 (e.g., via mechanical fasteners,an interference fit, or the like) and thus can rotate and translaterelative to the receiving portion 62 and the housing 60. Because theoperating member 100 extends through the cylindrical portion 66,however, the operating member 100 is constrained by the cylindricalportion 66 and prevented from moving in the radial direction R. Thereceiving portion 62 also includes at least one hole 68 that enables thecoupling device 70 to engage the operating member 100 as describedbelow.

The coupling device 70 of the end effector can be used to provideconstraints on longitudinal movement of the operating member. Thecoupling device 70 is disposed on the housing 60 and configured tocouple the operating member 100 to the housing 60 so as to permitrotation of the operating member 100 relative to the housing 60. In oneembodiment, the coupling device 70 includes a retaining member 72. Asdescribed below, the retaining member 72 is configured to engage theoperating member 100 to constrain movement of the operating member 100relative to the housing 60 in a longitudinal direction L of theoperating member 100 (shown in FIG. 4C) while permitting rotation of theoperating member 100.

As shown in FIGS. 5A-5C, the retaining member 72 includes a firstportion 74 and a second portion 76. The first portion 74 is configuredto translate in the longitudinal direction L and to rotate relative tothe housing 60. For example, as shown in FIG. 5A, the first portion 74is coupled to the release member 80 via a bearing 78 (e.g., a ballbearing). The first portion 74 is rigidly fixed to an inner race of thebearing 78 while an outer race of the bearing 78 is rigidly fixed to aslide member 82 of the release member 80, thus enabling the firstportion 74 to rotate with low friction relative to the housing 60. Theslide member 82 is connected to a knob 84 of the release member 80 andcan translate in the longitudinal direction L. A compression spring 86is disposed between the slide member 82 and a spring retainer 88 that isrigidly fixed to the housing 60. The compression spring 86 biases theslide member 82 and knob 84 toward a forward position (the connectposition shown in FIG. 5B). When the surgeon actuates the knob 84 bypulling the knob 84 back away from the housing 60 in the direction T1(into the release position shown in FIG. 5A), the slide member 82 moveswith the knob 84 and compresses the compression spring 86. Because thefirst portion 74 is coupled to the slide member 82 via the bearing 78,the first portion 74 also translates along the longitudinal direction Lwhen the knob 84 is moved into the release position. In this manner, therelease member 80 is coupled to the coupling device 70 and configured tomove the retaining member 72 between the connect position and therelease position.

The second portion 76 of the retaining member 72 is configured to movealong the radial direction R in response to movement of the firstportion 74 along the longitudinal direction L. As shown in FIGS. 5A and5B, the first portion 74 is disposed outward relative to the receivingportion 62 of the housing 60 and includes a surface 74 a configured toengage the second portion 76 and displace the second portion 76 in theradial direction R as the first portion 74 moves from a first position(the release position shown in FIG. 5A) to a second position (theconnect position shown in FIG. 5B). In one embodiment, as shown in FIG.5D, the first portion 74 is hollow cylinder, and the surface 74 a is aninclined inner surface of the cylinder. The first portion 74 is orientedrelative to the housing 60 such that the surface 74 inclines (i.e., aninner radius of the first portion 74 decreases) in the direction T1. Inthis embodiment, the second portion 76 comprises at least one ballbearing that is aligned with the hole 68 of the receiving portion 62.Preferably, the second portion 76 includes multiple ball bearings (e.g.,four), each aligned with a corresponding hole 68 on the receivingportion 62. Because the surface 74 a is inclined, the surface 74 apresses the ball bearings radially inward as the first portion 74 movesfrom the release position (FIG. 5A) to the connect position (FIG. 5B).Each ball bearing moves inward in the corresponding hole 68 along theradial direction R to engage a portion of the operating member 100.

The operating member 100 cooperates with the coupling device 70 tomaintain the constraints on longitudinal movement. The operating member100 includes a coupling region 102. When the coupling region 102 isaligned with the holes 68 and the coupling device 70 is moved to theconnect position, the coupling device 70 is adapted to constrainmovement of the operating member 100 in the longitudinal direction L toa region of constraint Y (shown in FIG. 5B). In this embodiment, thecoupling region 102 is a recess 104 in a peripheral surface 106 of theoperating member 100. As shown in FIG. 4C, the recess 104 has a proximalend 104 a and a distal end 104 b. The proximal and distal ends 104 a,104 b define a range of motion of the operating member 100 in the regionof constraint Y. For example, when the ball bearings move radiallyinward into the holes 68, they engage the recess 104 as shown in FIG.5B. When the coupling device 70 is in the connect position, the ballbearings are captured between the surface 74 a and the recess 104 andare therefore prevented from moving in the radial direction R.Similarly, because the ball bearings are received in the holes 68, theyare constrained from moving in the longitudinal direction L. Althoughthe ball bearings are captured, they are free to rotate in a mannersimilar to ball bearings in the bearing 78. Thus, the surface 74 afunctions as an outer race of a ball bearing while the recess 104functions as an inner race of a ball bearing. In this manner, the ballbearings (i.e., the second portion 76 of the retaining member 72) areconfigured to rotate relative to both the operating member 100 and thefirst portion 74 of the retaining member 72. In the connect position,when the ball bearings are engaged with the recess 104, the ballbearings interact with (i.e., contact) the proximal end 104 a and/or thedistal end 104 b of the recess 104 to constrain longitudinal movement ofthe operating member 100. In this embodiment, a longitudinal length L1of the recess 104 is sized such that the proximal and distal ends 104 a,104 b simultaneously contact the ball bearings when the ball bearingsare engaged with the first coupling region 102 in the connect position.As a result, the operating member 100 is substantially constrained frommoving in the longitudinal direction L.

As described above, both the first and second portions 74, 76 of theretaining member 72 can rotate freely, and the first portion 74 isslidable within the housing 60. In this manner, the retaining member 72is configured to rotate relative to the housing 60 and relative to theoperating member 100 and to move axially along the axis A-A of thehousing 60. Additionally, the retaining member 72 is configured to bemoveable between first and second positions (the connect and releasepositions) and is configured to constrain the operating member 100 whenthe retaining member 72 is in the first position (the connect positionof FIG. 5B) and permit decoupling of the operating member 100 from thehousing 60 when the retaining member 72 is in the second position (therelease position of FIG. 5A).

In the embodiment of FIGS. 4A-5B, the operating member 100 is a reamerfor resurfacing the acetabulum 22 during a hip replacement procedure.The operating member 100 includes the shaft 110 with proximal and distalends 100 a, 100 b. The proximal end 100 a is configured to engage thedrive motor 112. The distal end 100 b is a workpiece-engaging end thatincludes an attachment mechanism 114 that engages the cutting element116 that is configured to cut bone. In operation (as shown in FIG. 10A),the surgeon actuates the drive motor 112 with one hand and grasps theend effector 40 with the other hand to maneuver the end effector 40. Thedrive motor 112 imparts rotational motion to the operating member 100and the cutting element 116. As described further below in connectionwith step S8 of FIG. 11, the surgeon positions the operating member 100relative to the acetabulum 22 in accordance with a surgical plan andreams the surface of the acetabulum 22 with the rotating cutting element116. When the rotating cutting element 116 contacts the acetabulum 22,the surface of the acetabulum 22 (e.g., diseased bone) is cut away orresurfaced.

To provide flexibility to the surgeon, the end effector 40 is configuredsuch that the operating member 100 can be interchanged with otheroperating members. For example, the operating member 100 can beinterchanged with an operating member 200. In one embodiment, theoperating member 200 is an offset reamer. As is well known, an offsetreamer might be preferred over a straight reamer by a surgeon using anantero-lateral approach as opposed to a postero-lateral approach. Inthis embodiment, the operating member 200 is identical to the operatingmember 100 except the operating member 200 includes an offset portion220. For example, as shown in FIGS. 6A-6C, the operating member 200includes a proximal end 200 a configured to engage the drive motor 112and a distal end 200 b that includes an attachment mechanism 214 thatengages a cutting element (not shown) that is identical or similar tothe cutting element 116. The offset portion 220 is connected to a shaft210 and includes an offset shaft 224 having universal joints 228. Theoffset shaft 224 is enclosed by a support housing 222, and duplex pairball bearings 226 enable the offset shaft 224 to rotate relative to thesupport housing 222 with low friction. The offset portion 220 alsoincludes an anti-rotation pin 232 that engages a corresponding slot 632in the housing 60 of the end effector 40. The anti-rotation pin 232ensures the offset portion 220 is correctly assembled to the endeffector 40 and prevents rotation of the support housing 222 relative tothe housing 60 when torque is applied by the drive motor 112. Theoperating member 200 is coupled to the end effector 40 via the couplingdevice 70 in a manner identical to the operating member 100. Inparticular, the operating member 200 includes a coupling region 202having a recess 204 that engages the coupling device 70 of the endeffector 40 in the same manner described above in connection with theoperating member 100. In operation, the surgeon couples the shaft 210 ofthe operating member 200 to the end effector 40 (as described above inconnection with the operating member 100), attaches the knob 84, thedrive motor 112, and the cutting element 116 to the shaft 210, andoperates the operating member 200 in the same manner as the operatingmember 100.

The end effector 40 is also configured to be used individually andinterchangeably with multiple operating members having differentfunctions. For example, a first operating member can be configured tohave a first function, and a second operating member can be configuredto have a second function. In one embodiment, the first operating memberis the operating member 100 (shown in FIGS. 4A-5B) or the operatingmember 200 (shown in FIGS. 6A-6C) having a reaming function, and thesecond operating member is an operating member 300 (shown in FIGS.7A-8B) having an impaction function. In this embodiment, the operatingmember 300 is a straight impactor for implanting an acetabular cup(e.g., the acetabular cup 28 a) into a prepared acetabulum.Alternatively, the second operating member could be an operating member400 (shown in FIGS. 9A and 9B), such as an offset impactor. Theoperating member 300 is similar to the operating member 100 except theoperating member 300 is configured to engage with a prosthetic component316 (e.g., the acetabular cup 28 a) instead of a cutting element and animpactor head 312 instead of a drive motor. Additionally, the operatingmember 300 is configured to translate in the directions T1, T2 relativeto the end effector 40. Specifically, the operating member 300 isconfigured to translate relative to the coupled mounting portions 35, 50when the surgeon applies an impact force to the impactor head 312.

The operating member 300 includes a shaft 310 having a proximal end 300a and a distal end 300 b. The distal end 300 b is a workpiece-engagingend configured to couple to the prosthetic component 316 (e.g., viascrew threads). The proximal end 300 a is configured to withstand animpact force sufficient to impact the prosthetic device 316 into the hipjoint 10 of the patient. For example, the proximal end 300 a isconfigured to engage the impactor head 312 using any suitable mechanism(e.g., screw threads, mechanical fasteners, a key way, or the like). Asis well known, the impactor head 312 provides a surface 312 a that thesurgeon strikes (e.g., with a mallet 340) to impart force to theoperating member 300. The impactor head 312 can also be grasped by thesurgeon and used to rotate the operating member 300 to screw theprosthetic component 316 onto and off of the distal end 300 b.

The operating member 300 is coupled to the end effector 40 via thecoupling device 70 in a manner identical to that described above inconnection with the operating member 100 except the operating member 300is configured to translate relative to the end effector 40 when thecoupling device 70 is in the connect position. For example, as shown inFIGS. 8A and 8B, the operating member 300 includes a coupling region 302that engages the coupling device 70 of the end effector 40. In a manneridentical to the coupling region 102 of the first operating member 100,when the coupling region 302 is aligned with the holes 68 of thereceiving portion 62 and the coupling device 70 is moved to the connectposition (shown in FIG. 8B), the coupling device 70 constrains movementof the operating member 100 in the longitudinal direction L to a regionof constraint Z (shown in FIG. 8B). For example, the coupling region 302includes a recess 304 in a peripheral surface 306 of the shaft 310. Therecess 304 has a proximal end 304 a and a distal end 304 b. The proximaland distal ends 304 a, 304 b define a range of motion of the operatingmember 300 in the region of constraint Z. FIG. 8A shows the couplingdevice 70 in the release position. When the coupling device 70 moves tothe connect position (shown in FIG. 8B), the ball bearings (i.e., thesecond portion 76) of the retaining member 72 move radially inward intothe holes 68 and engage the recess 304. When the ball bearings areengaged with the recess 304, the ball bearings interact with (i.e.,contact) the proximal end 304 a and/or the distal end 304 b of therecess 304 to constrain longitudinal movement of the operating member300. In this embodiment, a longitudinal length L3 of the recess 304 issized such that the operating member can translate within the confinesof the recess 304. For example, the operating member 300 can translatein the direction T2 until the proximal end 304 a of the recess 304contacts the ball bearings thereby constraining movement of theoperating member 300 in the direction T2. Similarly, the operatingmember 300 can translate in the direction T1 until the distal end 304 bof the recess 304 contacts the ball bearings thereby constrainingmovement of the operating member 300 in the direction T1. The ability ofthe operating member 300 to translate passively in the region ofconstraint Z when the coupling device 70 is in the connect positionadvantageously allows the surgeon to strike the impactor head 312 withthe mallet 340 without the force of the mallet strikes being transmittedthrough the end effector 40 to the robotic arm 30. In this manner, thecoupling region 302 protects the robotic arm 30 from damage due toimpaction forces.

As can be seen by comparing FIGS. 5B and 8B, the longitudinal length L1of the recess 104 of the operating member 100 is less than thelongitudinal length L3 of the operating member 300. The longitudinallength L1 of the recess 104 and the interaction of the proximal anddistal ends 104 a, 104 b of the recess 104 with the retaining member 72(i.e., the ball bearings) define the region of constraint Y. Because theproximal and distal ends 104 a, 104 b simultaneously contact the ballbearings when the ball bearings are engaged with the first couplingregion 102 in the connect position, the region of constraint Y is asubstantially fixed axial location relative to the housing 60. As aresult, the operating member 100 is substantially constrained frommoving in the longitudinal direction L (i.e., the directions T1, T2)when the coupling device 70 is in the connect position. In contrast, theregion of constraint Z of the operating member 300 permits translationof the operating member 300. For example, the longitudinal length L3 ofthe recess 304 and the interaction of the proximal and distal ends 304a, 304 b of the recess 304 with the retaining member 72 (i.e., the ballbearings) define the region of constraint Z. Because the recess 304 iselongated, the ball bearings (of the retaining member 72) contact theproximal end 304 a of the recess 304, the distal end 304 b of the recess304, or neither when the coupling device 70 is in the connect position(i.e., when the retaining member 72 is engaged with the coupling region302). As a result, the region of constraint Z includes a first axiallocation (i.e., a location where the proximal end 304 a of the recess304 contacts the ball bearings) and a second axial location (i.e., alocation whether the distal end 304 b of the recess 304 contacts theball bearings), and the operating member 300 is moveable therebetween.In this manner, the coupling device 70 and the operating members 100,300 are configured to constrain the movement of the received operatingmember in the longitudinal direction L to a first region of constraint Ywhen the coupling device 70 engages the coupling region 102 of theoperating member 100 and to a second region of constraint Z, which isdifferent from the first region of constraint Y, when the couplingdevice 70 engages the coupling region 302 of the operating member 300.

The end effector 40 may also include a stop member 90 that is configuredto engage an operating member to limit movement of the operating memberrelative to the housing 60 and to provide an accurate axial location ofthe operating member 300 relative to the end effector 40. For example,as shown in FIGS. 7A and 7B, the stop member 90 includes a locatingsurface 92 (e.g., a counterbore) disposed within the housing 60 of theend effector 40 and a corresponding locating surface 94 (e.g., ashoulder or protrusion) disposed on the shaft 310 of the operatingmember 300. Although the stop member 90 may be disposed in any suitablelocation, in this embodiment, the stop member 90 is disposed remotelyfrom the recess 304 and the coupling device 70. In particular, the stopmember 90 is closer to the distal end 300 b of the operating member 300while the recess 304 and the coupling device 70 are closer to theproximal end 300 a of the operating member 300. When the locatingsurface 94 contacts the locating surface 92 (a seated position shown inFIG. 7A), translation of the operating member 300 in the direction T1 isprevented. In contrast, when the locating surfaces 92, 94 are not incontact (an extended position shown in FIG. 7B), the operating member300 can translate in the direction T1 and the direction T2 within theregion of constraint Z. In this manner, the stop member 90 is configuredto engage the operating member 300 to limit translation of the operatingmember 300 in the region of constraint Z. In one embodiment, the stopmember 90 is positioned so that the operating member 300 is preventedfrom translating within the full range of the region of constraint Z. Inthis embodiment, the operating member 300 translates between a firstlocation defined by the proximal end 304 a of the recess 304 and asecond location defined by the locating surface 94. In this manner, thestop member 90 can be used to effectively reduce the range of travel ofthe operating member 300 when the coupling device 70 is in the connectposition. Reducing the range of travel in this manner advantageouslyreduces contact stresses because the contact area between the locatingsurfaces 92, 94 is greater than the surface area between the ballbearings and the distal end 304 b of the recess 304.

In operation, after the surgeon finishes reaming the acetabulum 22, thesurgeon removes the operating member 100 (or the operating member 200)from the end effector 40. The surgeon couples the operating member 300(or the operating member 400) to the end effector 40 (in the same manneras described above in connection with the operating member 100) andconnects the prosthetic component 316 and the impactor head 312 to theoperating member 300. As shown in FIG. 10B, the surgeon grasps the endeffector 40 with one hand and uses the other hand to hold the mallet340. As described further below in connection with step S10 of FIG. 11,the surgeon properly positions the prosthetic component 316 relative tothe reamed acetabulum 22 and uses the mallet 340 to impart a force tothe surface 312 a of the impactor head 312. During impaction, the recess304 functions as a sliding passive joint that enables the operatingmember 300 to translate as described above. The impaction force impactsthe prosthetic component 316 onto the acetabulum 22. Between malletstrikes, the surgeon pushes the end effector 40 forward until thelocating surfaces 92, 94 of the stop member 90 are in contact (shown inFIG. 7A). The surgeon continues manually impacting the prostheticcomponent 316 until the prosthetic component 316 is implanted on theacetabulum 22 at the planned depth. After the prosthetic component 316is implanted, the surgeon grasps the impactor head 312 and rotates theoperating member 300 to unscrew the operating member from the implantedprosthetic component 316. If the acetabular cup includes a liner (e.g.,the liner 28 b), the surgeon then inserts the liner into the cup.

Depending on the position of the patient, instead of a straight impactor(e.g., the operating member 300), the surgeon may prefer to use anoffset impactor (e.g., the operating member 400). In one embodiment, theoperating member 400 (shown in FIGS. 9A and 9B) is similar to theoperating member 300 except the operating member 400 includes an offsetportion 420. For example, as shown in FIGS. 9A and 9B, the operatingmember 400 includes a proximal end 400 a configured to engage theimpactor head 312 and a distal end 400 b configured couple to theprosthetic component 316 (e.g., via screw threads). The offset portion420 is connected to a shaft 410 and includes an offset shaft 424 havinga universal joint 428 and a coupling knob 430. Because of the offset,the coupling knob 430 is used instead of the impactor head 312 toscrew/unscrew the operating member 400 to/from the prosthetic component316. An alternative embodiment could include two universal joints (e.g.,similar to the offset portion 220 of the operating member 200), whichwould enable the impactor head 312 to be used to screw/unscrew theoperating member 400 to/from the prosthetic component 316. One drawbackof this alternative configuration, however, is that it can addcomplexity and lower the strength of the offset shaft 424. The offsetshaft 424 is enclosed by a housing 422 and includes an anti-rotation pin432. The anti-rotation pin 432 engages the corresponding slot 632 in thehousing 60 to properly locate the offset portion 420 relative to the endeffector 40 and to prevent rotation of the housing 422 relative to thehousing 60. The operating member 400 is coupled to the end effector 40via the coupling device 70 in a manner identical to that described abovein connection with the operating member 300. In particular, theoperating member 400 includes a coupling region 402 having a recess 404that engages the coupling device 70 of the end effector 40 and enablesthe operating member 400 to translate longitudinally in the region ofconstraint Z. As shown in FIG. 11, the slot 632 is elongated therebyenabling the anti-rotation pin 432 to translate longitudinally in theslot 632 within the axial constraints of the region of constraint Z. Inoperation, the operating member 400 functions in the same manner as theoperating member 300 except the coupling knob 430 (instead of theimpactor head 312) is used to couple/decouple the prosthetic component316 to/from the operating member 400.

Surgical Application

In operation, the surgeon can use the robotic arm 30 to facilitate ajoint replacement procedure, such as reaming bone and implanting anacetabular cup for a total hip replacement or hip resurfacing procedure.As explained above, the robotic arm 30 includes a surgical toolconfigured to be coupled to a cutting element (for reaming) and toengage a prosthetic component (for impacting). For example, for reaming,the end effector 40 can couple to the operating member 100 or theoperating member 200, each of which couples to the cutting element 116.Similarly, for impacting, the end effector 40 can couple to theoperating member 300 or the operating member 400, each of which engagesthe prosthetic component 316. The robotic arm 30 can be used to ensureproper positioning during reaming and impacting.

FIG. 11 illustrates an embodiment of steps of a surgical procedure forperforming a total hip replacement. In this embodiment, steps S1-S7, S9,S11, and S12 can be performed in any known manner, with or withoutrobotic assistance. Steps S8 and S10 are preferably performed using therobotic arm 30. For example, step S8 (reaming) can be performed usingrobotic arm 30 with the end effector 40 coupled to the operating member100 or the operating member 200, and step S10 (impacting) can beperformed using the robotic arm 30 with the end effector 40 coupled tothe operating member 300 or the operating member 400.

Prior to the surgical procedure, a preoperative CT scan of the patient'spelvis 12 and femur 14 is obtained. As shown in FIG. 14A, the CT scan isused to create a three dimensional model 512 of the pelvis 12 and athree dimensional model 514 of the femur 14. The three dimensionalmodels 512, 514 are used by the surgeon to construct a surgical plan.Alternatively, X-ray images derived from the CT scan and/or the threedimensional models 512, 514 can be used for surgical planning, which maybe helpful to surgeons who are accustomed to planning implant placementusing actual X-ray images as opposed to CT based models. The surgeongenerates a surgical plan by designating a desired pose (i.e., positionand orientation) of the acetabular component 28 and the femoralcomponent 26 relative to the models 512, 514 of the patient's anatomy.For example, a planned pose 500 of the acetabular cup 28 a can bedesignated and displayed on a computer display, such as the displaydevice 9. During the surgical procedure, motion of the patient's anatomyand the surgical tool in physical space are tracked by the trackingdevice 8, and these tracked objects are registered to correspondingmodels in the navigation system 7 (image space). As a result, objects inphysical space are correlated to corresponding models in image space.Therefore, the surgical system 5 always knows the actual position of thesurgical tool relative to the patient's anatomy and the planned pose500, and this information is graphically displayed on the display device9 during the surgical procedure.

In step S1 of the surgical procedure, a cortical tracking array isattached to the femur 14 to enable the tracking device 8 to track motionof the femur 14. In step S2, the femur 14 is registered (using any knownregistration technique) to correlate the pose of the femur 14 (physicalspace) with models of the femur 14 in the navigation system 7 (imagespace) and the femur checkpoint is attached. In step S3, the femur 14 isprepared to receive a femoral implant (e.g., the femoral component 26)using a navigated femoral broach. In step S4, an acetabular trackingarray is attached to the pelvis 12 to enable the tracking device 8 totrack motion of the pelvis 12. In step S5, a checkpoint is attached tothe pelvis 12 for use during the surgical procedure to verify that theacetabular tracking array has not moved in relation to the pelvis 12.The checkpoint can be, for example, a checkpoint as described in U.S.patent application Ser. No. 11/750,807 (Pub. No. US 2008/0004633), filedMay 18, 2007, and hereby incorporated by reference herein in itsentirety.

In step S6, the pelvis 12 is registered (using any known registrationtechnique) to correlate the pose of the pelvis 12 (physical space) withmodel of the pelvis 12 in the navigation system 7 (image space). In oneembodiment, as shown in FIG. 14B, registration is accomplished using atracked probe to collect points on the pelvis 12 (physical space) thatare then matched to corresponding points on the representation 512 ofthe pelvis 12 (image space). In this embodiment, the display device 9shows the representation 512 of the pelvis 12, including one or moreregistration points 516. The registration points 516 help the surgeonunderstand where on the actual anatomy to collect points with thetracked probe. The registration points 516 can be color coded to furtheraid the surgeon. For example, a registration point 516 on the pelvis 12to be collected next with the tracked probe can be colored yellow, whileregistration points 516 that have already been collected can be coloredgreen and registration points 516 that will be subsequently collectedcan be colored red. After registration, the display device 9 can showthe surgeon how well the registration algorithm fit the physicallycollected points to the representation 512 of the pelvis 12. Forexample, as shown in FIG. 14C, error points 518 can be displayed toillustrate how much error exists in the registration between the surfaceof the representation 512 and the corresponding surface of the physicalpelvis 12. In one embodiment, the error points 518 can be color coded,for example, with error points 518 representing minimal error displayedin green and error points 518 representing increasing amounts of errordisplayed in blue, yellow, and red. As an alternative to color coding,error points 518 representing different degrees of error could havedifferent shapes or sizes. Verification points 519 can also bedisplayed. The verification points 519 illustrate to the surgeon whereto collect points with the tracked probe to verify the registration.When a registration point 519 is collected, the software of thenavigation system 7 displays the error (e.g., numerically inmillimeters) between the actual point collected on the anatomy and theregistered location of the representation 512 in physical space. If theregistration error is too high, the surgeon re-registers the pelvis 12by repeating the registration process of step S6.

In step S7, the robotic arm 30 is registered to correlate the pose ofthe robotic arm 30 (physical space) with the navigation system 7 (imagespace). The robotic arm 30 can be registered, for example, as describedin U.S. patent application Ser. No. 11/357,197 (Pub. No. US2006/0142657), filed Feb. 21, 2006, and hereby incorporated by referenceherein in its entirety.

In step S8, the surgeon resurfaces the acetabulum 22 using a reamer,such as the operating member 100 or the operating member 200, coupled tothe robotic arm 30. As described above in connection with the operatingmembers 100, 200, the surgeon couples the appropriate operating member(e.g., a straight or offset reamer) to the end effector 40, connects thecutting element 116 to the received operating member, and manuallymanipulates the robotic arm 30 (as shown in FIG. 10A) to ream theacetabulum 22. During reaming, the robotic arm 30 provides haptic (forcefeedback) guidance to the surgeon. The haptic guidance constrains thesurgeon's ability to manually move the surgical tool to ensure that theactual bone cuts correspond in shape and location to planned bone cuts(i.e., cuts consistent with the surgical plan).

Preferably, the constraint is adjusted to correspond to the surgicaltool, e.g., the cutting element 116, that is being used. In oneembodiment, the controller is programmed to generate force signals thatcause the force system to provide a first constraint (e.g., hapticguidance) on the surgeon's manual movement of the end effector 40 whenthe cutting element 116 is a first cutting element 116 a and provide asecond constraint (e.g., haptic guidance), different from the firstconstraint, on the surgeon's manual movement of the end effector 40 whenthe cutting element 116 is a second cutting element 116 b. As shown inFIG. 12, the first and second cutting elements 116 a, 116 b arehemispherical cutting elements configured to cut the acetabular bone,and a diameter D1 of the first cutting element 116 a is different from adiameter D2 of the second cutting element 116 b. In one embodiment, thediameter D1 of the first cutting element 116 a is less than a diameterD3 of the prosthetic component 316 by a predetermined amount, and thediameter D2 of the second cutting element 116 b is greater than thediameter D1 of the first cutting element 116 a. In an exemplaryembodiment, the predetermined amount is five millimeters less than thediameter D3 of the prosthetic component 316. In other embodiments, thepredetermined amount could be greater or less than five millimeters,such as two millimeters, three millimeters, seven millimeters, or arange (e.g., 5±2 millimeters).

Because the diameter D1 of the first cutting element 116 a is smallerthan the diameter D3 of the prosthetic component 316, the first cuttingelement 116 a can be used to make preliminary cuts, such as removingarticular cartilage and osteophytes. The preliminary cuts do not need tobe as accurate as the final cuts. Therefore, the preliminary cuts can bemade with a lesser degree of haptic constraint than the final cuts. Inparticular, when the first cutting element 116 a is used for reaming,the first constraint is configured to constrain, along a reference axisR-R, at least one point associated with the cutting element 116 a. Forexample, as shown in FIG. 13A, at least one point P on a central axisC-C of the cutting element 116 a can be constrained along the referenceaxis R-R such that the point P can move only along the reference axisR-R. In the embodiment of FIG. 13A, the point P is disposed on thecutting element 116 a. In other embodiments, the point P can be locatedon the central axis C-C of the cutting element 116 a without actuallyintersecting the cutting element 116 a. In this embodiment, thereference axis R-R is a desired axis of the prosthetic component 316when the prosthetic component 316 is implanted on the anatomy of thepatient. For example, the reference axis R-R can be a central axis ofthe prosthetic component 316 when the prosthetic component 316 isimplanted on the acetabulum 22 according to the surgeon's surgical plan.Thus, when reaming with the first cutting element 116 a, the robotic arm30 provides force feedback to constrain the surgeon's manual movement ofthe end effector 40 so that the point P stays on the reference axis R-R.In this manner, the trajectory of the surgical tool is constrained. Inone embodiment, the depth the point P can travel along the referenceaxis R-R is also constrained to prevent over reaming of the acetabulum22. Orientation of the end effector 40, however, is preferablyunconstrained when the first cutting element 116 a is used.

As reaming continues, progressively larger reamers are used. After thepreliminary cuts are made, the surgeon replaces the first cuttingelement 116 a with a larger cutting element, such as the second cuttingelement 116 b. When the second cutting element 116 b is coupled to theend effector 40, the robotic arm 30 applies the second constraint. Thesecond constraint is configured to constrain an orientation of thesurgical tool relative to the reference axis R-R. For example, as shownin FIG. 13B, the robotic arm 30 can apply force feedback to constrainthe surgical tool to maintain an axis of the surgical tool within apredefined angular distance θ from the reference axis R-R. The axis ofthe surgical tool can be, for example, the central axis A-A of thehousing 60 of the end effector 40, an axis of a shaft of the surgicaltool (e.g., a central axis B-B of the received operating member), or thecentral axis C-C of the cutting element 116 b. The predefined angulardistance θ is preferably 10 degrees but could be any other anglesuitable for the specific surgical application, such as, for example, 5degrees, 15 degrees, etc. Preferably, the robotic arm 30 applies boththe first and second constraints when the second cutting element 116 bis used. Thus, with the second cutting element 116 b, the robotic armconstrains both the trajectory and angular orientation of the surgicaltool. To avoid over reaming, the depth the surgical tool can travel canalso be constrained not to exceed a desired depth of the prostheticcomponent 316 when implanted on the acetabulum 22. In one embodiment,the second cutting element 116 b corresponds in size to the prostheticcomponent 316 and is used to make the final cut to the acetabulum 22. Inone embodiment, the controller is programmed to deactivate or shut offthe second cutting element 116 b when the shape of the final cutsubstantially corresponds to a desired shape of the final cut.

Reamers can be sized based on their outside diameter with reamer sizesprogressing in 1 millimeter increments. In one embodiment, for allcutting elements that are at least five sizes (e.g., five millimeters)below the size of the planned prosthetic component 316, the robotic arm30 applies the first constraint. In other words, if the diameter of acutting element is at least five millimeters less than the diameter D3of the prosthetic component 316, the cutting element can be used at anyangle but is constrained along the reference axis R-R. For largercutting elements (i.e., four sizes leading up to the size of the plannedcup), the robotic arm 30 additionally applies the second constraint sothat angular orientation of the surgical tool is also constrained. Theangular constraint may become progressively more restrictive as the sizeof the cutting element increases. In another embodiment, for reamersizes equal to two sizes below and two sizes above the size of theplanned prosthetic component 316, the robotic arm 30 applies both thefirst and second constraints. Preferably, the depth of travel of thesurgical tool is constrained to prevent reaming beyond the planned depthof the prosthetic component 316.

The first and second constraints are preferably activated by thecontroller that controls the force system of the robotic arm 30. Forexample, the controller can be programmed to generate control signalsthat cause the force system to provide at least one of the firstconstraint and the second constraint when a portion of the cuttingelement 116 (e.g., the first cutting element 116 a or the second cuttingelement 116 b) coincides with an activation region 510. The activationregion 510 (shown in FIGS. 14A and 14D) can be a virtual region that isdefined relative to the anatomy of the patient. For example, theactivation region 510 can be defined relative to the planned pose 500 ofthe prosthetic component 316. In one embodiment, the activation region510 coincides with the boundary of the planned pose 500 and thereforehas the same shape and location as the planned pose 500. In anotherembodiment, at least a portion of the planned pose 500 and theactivation region 510 coincide. The activation region 510 can alsoextend beyond a boundary of the planned pose 500. For example, as shownin FIGS. 14A and 14D, the activation region 510 is a cylindrical volumethat extends beyond the boundary of the planned pose 500. Preferably,the cylindrical volume is coaxial with an axis of the planned pose 500,such as the central axis C-C of the prosthetic component 316 when theprosthetic component 316 is implanted on the anatomy in the planned pose500.

During surgery, a representation of the surgical tool is displayed onthe display 9 relative to the planned pose 500, the activation region510, and/or the representations 512, 514 of the anatomy, as shown inFIG. 14D. The representation is a graphical model in image space thatcorresponds to the actual surgical tool in physical space (viaregistration of the robot arm 30 in step S7) and includes arepresentation 520 a of the shaft of the received operating member and arepresentation 520 b of the cutting element 116. The surgeon uses thisview to manually navigate the surgical tool into the incision. In oneembodiment, the surgeon can freely move the surgical tool until aportion of the surgical tool intersects the activation region 510 atwhich time the force system controls the robotic arm 30 to provide theappropriate constraint (e.g., the first constraint and/or the secondconstraint). In one embodiment, the appropriate constraint activateswhen the representation 520 a of the shaft of the received operatingmember is completely bounded by the cylindrical volume of the activationregion 510, as shown in FIG. 14D. Once the appropriate constraints areactive, the activation region 510 can be removed from the displayedimage, as shown in FIG. 14E.

The first and second constraints ensure that the bone cuts to theacetabulum accurately correspond to the bone cuts of the planned pose500 of the prosthetic component 316. Because the first and secondconstraints are applied by actuators, the first and second constraintsare not infinite. Accordingly, the surgeon may be able to override thefirst and second constraints by manually moving the end effector 40 withsufficient force to overcome the force feedback provided by the roboticarm 30. To avoid damage to the patient and/or inaccurate bone cuts, thecontroller is preferably programmed to automatically control at leastone aspect of the pose of the surgical tool if the surgeon manuallyoverrides the first constraint and/or the second constraint. Forexample, during reaming, if the surgeon pushes the end effector 40 suchthat the shaft of the received operating member exceeds the predefinedangular distance θ from the reference axis R-R, the robotic arm 30 canapply active force feedback to try to move the shaft of the receivedoperating member back within the predefined angular distance θ. Anotheroption is for the controller to deactivate or shut off the reamer if thefirst constraint and/or the second constraint is overridden by thesurgeon.

In step S9, the surgeon verifies that the registration (i.e., thegeometric relationship) between the acetabular tracking array and thepelvis 12 is still valid by contacting the pelvis checkpoint with atracked probe as described, for example, in U.S. patent application Ser.No. 11/750,807 (Pub. No. US 2008/0004633), filed May 18, 2007, andhereby incorporated by reference herein in its entirety. If registrationhas degraded (e.g., because the acetabular tracking array was bumpedduring reaming), the pelvis 12 is re-registered. Registrationverification can be performed any time the surgeon wants to check theintegrity of the acetabular registration.

In step S10, the prosthetic component 316 is implanted on the reamedacetabulum 22 using an impactor tool, such as the operating member 300or the operating member 400, coupled to the robotic arm 30. As describedabove in connection with the operating members 300, 400, the surgeonremoves the reamer from the end effector 40, connects the appropriateoperating member (e.g., a straight or offset impactor) to the endeffector 40, and attaches the prosthetic component 316 (e.g., theacetabular cup 28 a) to the operating member. The surgeon then manuallymanipulates the robotic arm 30 (as shown in FIG. 10B) to impact theprosthetic component 316 on the acetabulum 22. While the surgeon impactsthe prosthetic component 316, the robotic arm 30 provides hapticguidance, based on the surgical plan, that constrains the surgeon'sability to move the surgical tool to ensure that the actual pose of theprosthetic component 316 that is coupled to the surgical toolsubstantially corresponds to the planned pose 500 when the prostheticcomponent 316 is impacted onto the acetabulum 22. In one embodiment, thecontroller is programmed to compare a target pose (e.g., the plannedpose 500 and/or the activation region 510) of the prosthetic component316 and an actual pose of the prosthetic component 316 engaged by thesurgical tool and to generate control signals that cause the forcesystem to allow movement of the surgical tool within a range of movementand provide haptic feedback to constrain the surgeon's ability tomanually move the surgical tool beyond the range of movement. The rangeof movement can be defined, for example, relative to a desired aspect ofthe prosthetic component 316 when the prosthetic component 316 isimplanted on the anatomy, such as an angle (e.g., a version angle, aninclination angle), an axis, an orientation, a center of rotation, aboundary, and/or a depth. The haptic feedback can then resist movementof the surgical tool by the surgeon that would cause substantialdeviation between at least one aspect of the actual pose of theprosthetic component 316 and a corresponding desired aspect of thetarget pose of the prosthetic component 316. The haptic feedback can beapplied as the surgeon is moving the prosthetic component 316 toward theimplantation site and is preferably maintained as the surgeon implantsthe prosthetic component 316 on the anatomy. As a result, the acetabularcup 28 a can be implanted on the acetabulum 22 such that the inclinationaccuracy, version accuracy, and center of rotation of the acetabular cup28 a substantially correspond to the surgical plan.

In a manner identical to that described above in connection with step S8(reaming), during the impaction step S10, the display device 9 can showthe planned pose 500, the activation region 510, the representations512, 514 of the anatomy, and a representation of the surgical tool.During impaction, however, the representation 520 b represents theprosthetic component 316 as opposed to the cutting element 116.Additionally, as described above in connection with step S8, thecontroller can activate the haptic feedback during the impactionprocedure when at least a portion of the actual pose of the surgicaltool coincides with at least a portion of the activation region 510 ofthe target pose. Also as described above in connection with step S8, ifthe surgeon moves the end effector 40 to override the haptic feedback,the controller can initiate automatic control of the surgical tool tosubstantially align at least one aspect of the actual pose with thecorresponding desired aspect of the target pose.

In step S11, the surgeon installs the femoral component 26 on the femur14, and in step S12, the surgeon determines leg length and femoraloffset. As shown in FIG. 14F, the display device 9 can display arepresentation 522 of the implanted acetabular component 28 and arepresentation 524 of the implanted femoral component 26. Additionally,as shown in FIG. 14G, at any time during the surgical procedure, thedisplay device 9 can show data related to progress and/or outcome. Forexample, after reaming in step S8 and/or impacting in step S10), data525 relating to the actual position of the reamed acetabulum 22 (or theimplanted acetabular cup 28 a) can include, for example, numerical datarepresenting error between the actual and planned locations in the threeorthogonal planes of the patient's anatomy (i.e., medial/lateral,superior/inferior, and anterior/posterior).

Parking Configuration

The surgical system 5 preferably is configured to park or hold therobotic arm 30, for example, during a surgical procedure when thesurgeon is not actively using the robotic arm 30 to perform a task. Theparking configuration applies to a moveable member of the robotic arm30, such as the articulated arm 34 or an instrumented linkage that isused to track an object (e.g., a mechanical tracking arm) as described,for example, in U.S. Pat. No. 6,322,567, which is hereby incorporated byreference herein in its entirety. In the parking configuration, themoveable member is secured in a safe position, and the working end ofthe moveable member (e.g., the surgical tool) is prevented from driftingoutside the sterile field of the surgical procedure.

The surgical system 5 preferably is configured to account for differentweights of objects connected to the robotic arm 30. As explained above,the robotic arm 30 is configured to permit a user (e.g., the surgeon) tomanually move the articulated arm 34 to permit an object coupled to thearticulated arm 34 (e.g., the received operating member) to bemanipulated in space and thereby facilitate the performance of a task(e.g., bone cutting, implant impaction) using the coupled object. Thearticulated arm 34 is adapted to couple to multiple interchangeableobjects, such as a first object and a second object. The first objectcould be, for example, the operating member 100 or the operating member200, and the second object could be the operating member 300 or theoperating member 400 (or vice versa). Because the operating members 100,200, 300, 400 have different configurations and functions, they may alsohave substantially different weights. For example, in one embodiment, aweight of the second object is at least three times greater than aweight of the first object. In another embodiment, a weight of thesecond object is at least thirty-six percent greater than a weight ofthe first object. In another embodiment, a weight of the second objectis at least fifty-two percent greater than a weight of the first object.In another embodiment, a weight of the second object is at leastninety-four percent greater than a weight of the first object. Inanother embodiment, a weight of the operating member 100 is about 360grams, a weight of the operating member 200 is about 460 grams, a weightof the operating member 300 is about 490 grams, and a weight of theoperating member 400 is about 700 grams. Thus, the parking configurationis configured to accommodate payloads of the robotic arm 30 that havesubstantially different weights.

The parking configuration can be achieved using a brake. In operation,the brake limits manual movement of at least a portion of the moveablemember. For example, the brake limits manual movement of at least aportion of the articulated arm 34 to inhibit manipulation in space ofthe coupled object. Because the articulated arm 34 is used with multipleoperating members during a single surgical procedure, the brake shouldwork both when the articulated arm 34 is coupled to the first object andwhen the articulated arm 34 is coupled to the second object withoutrequiring mechanical reconfiguration of the brake, which would disruptsurgical workflow. The brake can be implemented using any suitablecombination of mechanical and/or electrical components. In oneembodiment, the brake is a virtual brake. In contrast to a physicalbrake, the virtual brake does not include conventional mechanical brakecomponents. Instead, as explained below, the virtual brake isimplemented using the controller and the force system of the robotic arm30.

In one embodiment, the virtual brake is implemented by controlling oneor more actuators of the force system to apply a holding torque (i.e., abraking force) to one or more joints of the articulated arm. Applicationof the holding torque is based on a position of the articulated arm 34relative to a braking region where the brake is configured to engagewhen the surgeon moves at least a portion of the articulated arm 34(such as one or more joints) into the braking region. In particular, thebrake is configured to apply the braking force only if the joint (orjoints) is in the braking region. For example, the brake can beconfigured to limit manual movement of the joint (or joints) based onthe braking region, which can be defined, for example, by a position ofthe joint (or joints). In one embodiment, the braking region is adefined angular range of motion α of a joint J of the articulated arm 34and can also include angular ranges of motion of other joints of thearticulated arm 34. The angular range of motion α can be any range ofmotion that places the articulated arm 34 in a desired parkingconfiguration. For example, FIG. 15 shows the joint J in a substantiallyhorizontal position. In this embodiment, to park the joint J in asubstantially vertical position, the joint J is moved from thehorizontal position to the vertical position. In one embodiment, theangular range of motion α of the substantially vertical position isabout +/−15 degrees. In other embodiments, the angular range of motion αcan be, for example, about +/−15 degrees, about +/−30 degrees, or about180+/−30 degrees. In one embodiment, the most distal joint can have anangular range of motion α of about +/−30 degrees or about 180+/−30degrees. The brake is configured to engage (and hold the joint J in thebraking region) when the surgeon moves the joint J into the brakingregion. For example, the surgeon moves the joint J from the horizontalposition approximately 90 degrees in a direction V until the position ofthe joint J is within the angular range of motion α. When the positionof the joint J is within the angular range of motion α, the controllergenerates a signal that controls the actuator of the joint J to applythe holding torque. When the holding torque is applied, the brake isengaged and the joint J stays locked in position. When the joint J movesinto this braking region, the controller preferably also generatessignals that control the actuators of one or more other joints of thearticulated arm 34 to apply holding torque resulting in a plurality ofbraked joints. As a result, the overall position of the articulated arm34 is locked in the parking configuration. The surgeon can then safelyleave the articulated arm 34 unattended, change the operating member, orperform any other task without worrying that the surgical tool willdrift outside the sterile field or interfere with the patient or otherequipment in the operating room. Although the above description resultsin engagement of the brake when the joint J is within the angular rangeof motion α, the brake can also be configured to engage only if multiplejoints are moved within their respective angular ranges of motion.

Disengagement of the brake can also be based on the braking region. Inone embodiment, the brake is configured to disengage when the surgeonmoves at least one of the braked joints (such as the joint J) outsidethe braking region of that particular joint. For example, to disengagethe brake, the surgeon moves the articulated arm 34 with sufficientforce to overcome the applied holding torque or braking force of thejoint J. The magnitude of the braking force is small enough to enablethe surgeon to manually move the articulated arm 34 to overcome thebraking force. The braking force can be adjusted for a particularsurgeon and/or a particular surgical procedure, and different joints canhave different braking forces. For example, the braking force can be ina range of about 5 to 12 Nm. For example, in one embodiment, the firstjoint (i.e., the most proximal joint) can have a braking force of about6 Nm, the second joint can have a braking force of about 12 Nm, thethird joint can have a braking force of about 9 Nm, the fourth joint canhave a braking force of about 11 Nm, the fifth joint can have a brakingforce of about 7 Nm, and the sixth joint (i.e., the most distal joint)can have a braking force of about 5 Nm. When the joint J moves outsidethe braking region into an unbraked region 610, the controller generatesa signal that controls the actuator of the joint J (and any other brakedjoints) to discontinue application of the holding torque. The surgeoncan then freely move the articulated arm 34. In one embodiment, anoverlap exists between the braking region and the unbraked region 610 toprevent accidental release of the brake. Additionally or alternatively,the brake can be configured to disengage independent of the brakingregion. For example, if the articulated arm 34 includes one or morebraked joints and at least one unbraked joint, such as a wrist joint W,the brake can be configured to disengage when the surgeon manually movesthe unbraked joint, for example, by twisting the wrist joint W. Althoughthe above description results in disengagement of the brake when thejoint J is moved outside the angular range of motion α, the brake canalso be configured to disengage only if multiple joints are movedoutside their respective angular ranges of motion.

One advantage of the holding torque embodiment is that the brake isimplemented in joint space, and each individual joint actuator can havea unique holding torque limit. For example, a heavier joint may requirea larger holding torque because, in addition to braking, the holdingtorque also has to compensate for gravitational forces acting on thejoint. In contrast, a light weight joint can have a relatively smallholding torque because the lighter joint requires less gravitycompensation. This distinction can be used to facilitate disengagementof the brake. In particular, because it is easier for the surgeon tomanually move a joint that has a lower holding torque, movement oflighter joints can be used to trigger disengagement of the virtualbrake.

In another embodiment, the virtual brake is implemented in Cartesianspace using a haptic object. The haptic object embodiment is similar tothe holding torque embodiment except the braking region is defined by ahaptic object instead of an angular range of motion of a joint. Asexplained in U.S. patent application Ser. No. 11/357,197 (Pub. No. US2006/0142657), filed Feb. 21, 2006, which is hereby incorporated byreference herein in its entirety, a haptic object is a virtual objectdefined by a mapping between force and/or torque (i.e., force feedback)and position. The haptic object is registered to physical space anddefines a virtual boundary in physical space. The haptic object can bedefined so that the virtual boundary has any desired size, shape, andlocation appropriate for a particular surgical procedure. In a mannersimilar to a virtual cutting boundary activated during bone cutting,movement of a specified portion of the articulated arm 34 beyond thevirtual boundary is constrained by force feedback applied by the forcesystem. When a haptic object is used as a virtual brake, the hapticobject functions as “virtual holster” for the surgical tool or otherequipment attached to the end of the articulated arm 34, and the forcefeedback applied by the force system is the braking force. As shown inFIGS. 16A and 16B, the virtual holster includes a virtual boundary 600(a first region) and an interior region 605 (a second region) that isbounded by the virtual boundary 600. When the surgeon manually moves thesurgical tool inside the virtual boundary 600, the controller controlsthe force system to constrain motion of the articulated arm 34 such thatthe surgical tool is maintained within the virtual boundary 600 of thevirtual holster.

In the holding torque embodiment, the braking force is substantiallycontinuous in the braking region because a constant holding torque isapplied regardless of the position of the joint J within the brakingregion. As a result, the articulated arm 34 has a smooth continuous feelas the surgeon moves the joint J in the braking region. In contrast, inthe haptic object embodiment, the braking force is substantiallydiscontinuous in the braking region because the braking force istypically applied at the virtual boundary 600 of the haptic object butnot within the interior region 605 of the haptic object. For example, inone embodiment, force feedback is applied only at or near the virtualboundary 600 but not in the interior region 605. Thus, when the surgicaltool is parked in the virtual boundary 600, the surgical tool can driftfreely within the confines of the virtual boundary 600 but is preventedfrom drifting outside the virtual boundary 600. In this manner, thebraking region includes a first region (i.e., the virtual boundary 600)in which a braking force is applied and a second region (i.e., theinterior region 605) in which the braking force is not applied.Alternatively, the mapping of the haptic object can be defined such thatforce feedback is applied in the interior region 605 as well as at ornear the virtual boundary 600 so that the surgical tool does not driftwithin or beyond the virtual boundary 600.

As described above in connection with the holding torque embodiment, inthe haptic object embodiment, the brake is configured to engage when thesurgeon manually moves the surgical tool (or other specified portion ofthe articulated arm 34) into the braking region. For example, when thesurgeon moves the articulated arm 34 from the location shown in FIG. 16Ato the location shown in FIG. 16B, the surgical system 5 detects when aspecified portion of the surgical tool (such as the tip and/or shaft) iswithin the virtual boundary 600, and the controller engages the brake.In the haptic object embodiment, engaging the brake can include anaffirmative action, such as turning on the force feedback of the hapticobject when the surgical tool is within the virtual boundary 600.Alternatively, the force feedback of the haptic object can becontinuously active so that engaging the brake includes the surgicalsystem 5 determining that the surgical tool is within the virtualboundary 600. In this manner, the controller is programmed to determinewhether at least a portion of the articulated arm 34 is in a definedbraking region and generate a signal configured to cause a definedbraking force to be applied to inhibit movement of at least the portionof the articulated arm 34 when at least the portion of the articulatedarm 34 is determined to be in the braking region. Disengagement of thebrake can also be based on the braking region. In one embodiment, thebrake is configured to disengage when the surgeon moves the surgicaltool (or other specified portion of the articulated arm 34) outside thebraking region. For example, to disengage the brake, the surgeon movesthe articulated arm 34 with sufficient force to overcome the forcefeedback applied by the force system at the virtual boundary 600. Whenthe surgical tool moves outside the virtual boundary 600 into theunbraked region 610, the brake is disengaged. In the haptic objectembodiment, disengaging the brake can include an affirmative action,such as turning off the force feedback of the haptic object after thesurgical tool has moved outside the virtual boundary 600. Alternatively,the force feedback of the haptic object can be continuously active sothat disengaging the brake includes the determination by the surgicalsystem 5 that the surgical tool is outside the virtual boundary 600. Aswith the holding torque embodiment, the magnitude of the braking forceis small enough to enable the surgeon to manually move the articulatedarm 34 to overcome the braking force. In one embodiment, an overlapexists between the braking region and the unbraked region 610 to preventaccidental release of the brake. Additionally or alternatively, thebrake can be configured to disengage independent of the braking region,such as by twisting the wrist joint W as explained above in connectionwith the holding torque embodiment.

The parking configuration can be used with any moveable member of therobotic arm 30 or with a moveable member that is not associated with therobotic arm 30. For example, the moveable member can be an instrumentedlinkage system for surgical navigation. In one embodiment, as shown inFIG. 17A, the instrumented linkage system includes an instrumentedlinkage (or articulated member) 800 having a plurality of linksconnected by a plurality of moveable joints. As is well known, thejoints are instrumented (e.g., using joint encoders) to enablemeasurement of the coordinates of a proximal link 805 relative to adistal link 810. When the distal link 810 is connected to an object tobe tracked (such as a bone), the pose of the tracked object can bedetermined. As the tracked object moves, the instrumented linkage 800moves along with the tracked object. In this manner, the pose of thetracked object can be tracked as the tracked object moves in physicalspace. One advantage of using an instrumented linkage system fortracking is that the instrumented linkage system enables surgicalnavigation without having a line-of-sight constraint. In contrast, anoptical tracking system requires a line of sight between an opticalcamera and trackable markers disposed on the tracked object.

During a surgical procedure, the instrumented linkage 800 can beconfigured to be disposed in a parking configuration where theinstrumented linkage 800 is secured in a safe position and is preventedfrom drifting outside the sterile field of the surgical procedure. Asdescribed above in connection with the articulated arm 34, the parkingconfiguration for the instrumented linkage 800 can be achieved using abrake. The brake can be implemented using a virtual brake (e.g., asdescribed above) or a physical brake. In one embodiment, a joint J2 iscoupled with an actuator either directly or through cabling such thatthe instrumented linkage 800 is back-drivable. During normal operation,the actuator can apply a torque to compensate for a gravity load due tothe weight of the instrumented linkage 800. The parking configuration(shown in FIG. 17B) can be achieved by applying a holding torque to atleast one joint of the instrumented linkage 800. In one embodiment, theparking configuration can be implemented with a physical brake mechanism820 using any suitable combination of electro or/and mechanical brakes,electrorheological (ER) or magnetorheological (MR) fluid brakes, and/orthe like. As described above in connection with the articulated arm 34,application of the brake is preferably based on a position of theinstrumented linkage 800 relative to a braking region. In a preferredembodiment, the brake mechanism 820 is configured to apply a brakingforce only if a joint (or joints) of the instrumented linkage 800 is inthe braking region. For example, the brake can be configured to engage(based on a signal from the controller) when the joint J2 and a joint J4are each within a pre-defined angular range of motion, for example, asdescribed above in connection with the articulated arm 34. The angularrange of motion can be, for example, 10 degrees. Disengagement of thebrake can be also based on the breaking region. In one preferredembodiment, the brake is configured to disengage when the surgeon movesat least one of the joints outside of the braking region. For example,the brake can be configured to disengage when the surgeon moves thejoint J4 outside the range of motion of the braking region. In thismanner, a surgical system can include an articulated member configuredto be connected to an object to be tracked and a controller programmedto determine whether at least a portion of the articulated member is ina defined braking region and generate a signal configured to cause abraking force to be applied to inhibit movement of at least the portionof the articulated member when at least the portion of the articulatedmember is determined to be in the braking region.

Preferably parameters of the virtual brake can be adjusted depending oncircumstances and/or desired configurations. According to an embodiment,the virtual brake is defined by a virtual brake configuration thatincludes parameters such as the braking force, a size of the brakingregion, a location of the braking region, and/or a shape of the brakingregion. As explained above in connection with the holding torque andhaptic object embodiments, these parameters of the can be tailored for aparticular surgical application. Additionally, the controller can beprogrammed to enable the surgeon to continuously control the virtualbrake configuration. For example, before, during, and/or after asurgical procedure, the surgeon can use a computer (such as a computeron the navigation system 7) to adjust one or more of the parameters ofthe virtual brake configuration. In this manner, the controller isprogrammed to enable the surgeon to continuously modify the parametersof the virtual brake configuration. Advantageously, the virtual brakeconfiguration can be modified without changing a mechanicalconfiguration of the robotic arm 30. For example, the actuators of theforce system are capable of applying varying levels of holding torqueand force feedback. Thus, to modify the braking force, the controllersimply needs to control the actuators to output a different magnitude ofholding torque or force feedback. Similarly, to modify the brakingregion, the controller simply needs to be provided with new values forthe angular range of motion α, the size of the virtual boundary 600, thelocation of the virtual boundary 600, and/or the shape of the virtualboundary 600. Thus, the virtual brake configuration can be modified atany time. For example, if an operating member that is extremely heavy isgoing to be coupled to the articulated arm 34, the surgeon may want toincrease the braking force to ensure the brake can safely hold the heavyoperating member. Similarly, for operating members having differentfunctions, the surgeon may prefer braking regions in differentlocations. Although the virtual brake configuration can be modified atany time, for a particular surgical procedure, to enable continuoussurgical workflow, it is preferable to have a brake that has the sameconfiguration regardless of what object is coupled to the articulatedarm 34. This can be accomplished by setting the parameters of thevirtual brake configuration to ensure that the brake can safelyaccommodate all objects that will be coupled to the articulated arm 34during the surgical procedure.

One skilled in the art will realize the invention may be embodied inother specific forms without departing from the spirit or essentialcharacteristics thereof. The foregoing embodiments are therefore to beconsidered in all respects illustrative rather than limiting of theinvention described herein. Scope of the invention is thus indicated bythe appended claims, rather than by the foregoing description, and allchanges that come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

What is claimed is:
 1. A surgical system, comprising: a surgical toolconfigured to be coupled to a cutting element; a force system configuredto provide at least some force to the surgical tool; and a controllerprogrammed to generate control signals that cause the force system to:provide a first constraint on a user's manual movement of the surgicaltool in one degree of freedom when the cutting element is a firstcutting element; and provide a second constraint, in addition to thefirst constraint, on the user's manual movement of the surgical toolwhen the cutting element is a second cutting element, wherein the secondconstraint comprises a constraint in a second degree of freedom, therebyconstraining the user's manual movement of the surgical tool in twodegrees of freedom when the cutting element is a second cutting element.2. The surgical system of claim 1, wherein a diameter of the firstcutting element is different from a diameter of the second cuttingelement.
 3. The surgical system of claim 1, wherein the first and secondcutting elements are each configured to cut a bone to receive aprosthetic component, and wherein a diameter of the first cuttingelement is less than a diameter of the prosthetic component by apredetermined amount and a diameter of the second cutting element isgreater than the diameter of the first cutting element.
 4. The surgicalsystem of claim 3, wherein the predetermined amount is five millimetersless than the diameter of the prosthetic component.
 5. The surgicalsystem of claim 1, wherein the first constraint is configured toconstrain, along a reference axis, at least one point associated withthe cutting element.
 6. The surgical system of claim 5, wherein thereference axis is a desired axis of a prosthetic component when theprosthetic component is implanted on an anatomy of a patient.
 7. Thesurgical system of claim 5, wherein the at least one point is a point ona central axis of the cutting element.
 8. The surgical system of claim5, wherein the at least one point is disposed on the cutting element. 9.The surgical system of claim 1, wherein the second constraint isconfigured to constrain an orientation of the surgical tool relative toa reference axis.
 10. The surgical system of claim 9, wherein thereference axis is a desired axis of a prosthetic component when theprosthetic component is implanted on an anatomy of a patient.
 11. Thesurgical system of claim 9, wherein the second constraint is configuredto constrain the surgical tool to maintain an axis of the surgical toolwithin a predefined angular distance from the reference axis.
 12. Thesurgical system of claim 11, wherein the axis of the surgical tool is atleast one of an axis of a shaft of the surgical tool and a central axisof the cutting element.
 13. The surgical system of claim 1, wherein thecontroller is further programmed to generate control signals that causethe force system to provide at least one of the first constraint and thesecond constraint when a portion of at least one of the first cuttingelement and the second cutting element coincides with an activationregion.
 14. The surgical system of claim 13, wherein the activationregion is defined relative to a planned pose of a prosthetic componentimplanted on an anatomy of a patient.
 15. The surgical system of claim14, wherein at least a portion of the activation region at least one ofextends beyond a boundary of the planned pose and coincides with atleast a portion of the planned pose.
 16. The surgical system of claim 1,wherein the controller is further programmed to automatically control atleast one aspect of a pose of the surgical tool if the user manuallyoverrides at least one of the first constraint and the secondconstraint.
 17. The surgical system of claim 1, wherein the firstconstraint is configured to constrain the user's manual movement of thesurgical tool as the user makes a preliminary cut with the first cuttingelement and the second constraint is configured to constrain the user'smanual movement of the surgical tool as the user makes a final cut withthe second cutting element.
 18. The surgical system of claim 17, whereinthe controller is programmed to deactivate the second cutting elementwhen a shape of the final cut substantially corresponds to a desiredshape of the final cut.